Asthma is an inflammatory disease characterized by airway hyperreactivity. It is now clear that the relationship that exists between the inflammation and the increase in airway reactivity is one of cause and effect. As such, the delineation and subsequent control of the underlying inflammatory events are central to the amelioration of asthma. Accumulated evidence has defined a prominent role for eosinophils and, in particular, major basic protein (MBP) in the pathogenesis of asthma. Although the cytotoxicity of high concentrations of MBP has been cited most frequently as the basis for the pathogenic effects, additional evidence now indicates that lesser concentrations of MBP initiate noncytotoxic responses in a variety of cells implicated in the pathogenesis of asthma. Specifically, studies by the Principal Investigator have established that MBP, in concentrations similar to the levels detected in sputum and bronchoalveolar lavage fluid of asthmatics, stimulates basophil secretion as well as neutrophil release of superoxide anion and specific granule contents. The MBP is unique among other eosinophil cationic proteins in this action. Further, MBP had a synergistic action on neutrophil superoxide anion release stimulated by platelet-activating factor, which is also implicated in the pathogenesis of asthma. Given the well documented roles of neutrophils and basophils in allergic late-phase inflammatory events, it is postulated that the activation of these cells by MBP may precede or occur in the absence of direct epithelial damage and thus constitute important amplification mechanisms in the pathogenesis of asthma. This potential is enhanced by recent findings interleukins and 5 potentiate basophil responsiveness to MBP. This proposal has three goals. (1) Distinguish between a receptor-mediated and a receptor-independent mechanism for the noncytotoxic actions of MBP and also define the relationship between this action and the cytotoxic properties of the molecule. (2) Define the signal events for MBP-induced activation of basophils and neutrophils and also establish the basis for the differences observed in the MBP-induced activation events for each. (3) Using peptides and site-directed mutagenesis of recombinant granule MBP, determine the structure-activity requirements for both the noncytotoxic and cytotoxic properties of MBP. Together, these findings will identify pharmacologic actions to control the pathogenic activities of MBP in asthma and other allergic late-phase reactions.